This invention relates to a light-emitting semiconductor device, or light-emitting diode (LED) according to more common parlance, and more particularly to such devices having active layers made from chemical compounds such for example as aluminum gallium arsenide (AlGaAs), aluminum gallium indium phosphide (AlGaInP), gallium nitride (GaN), aluminum gallium indium nitride (AlGaInN), and derivatives thereof. The invention also concerns a method of making such light-emitting semiconductor devices.
Compound semiconductors containing AlGaInP, for instance, represent familiar materials for the active layers of light-emitting semiconductor devices. An example of such devices has a substrate of gallium arsenide (GaAs) on which there are laminated a plurality of active semiconductor layers composed primarily of AlGaInP. The AlGaInP semiconductor layers are relatively easy to grow on the GaAs substrate by epitaxy.
One of the problems encountered with this conventional light-emitting device is that the GaAs substrate is highly absorptive of the light of the wavelength range emitted by the active semiconductor layers or the main semiconductor region of the light-emitting device. Much of the light that has issued from the active layers toward the substrate has been absorbed thereby, running counter to the objective of making the light-emitting device as high as feasible in efficiency.
A known remedy to this problem was to remove the GaAs substrate after epitaxially growing the active semiconductor layers thereon. A transparent support substrate of gallium phosphide (GaP) or the like, different from the removed growth substrate which had been used for epitaxial growth of the active semiconductor layers, was then bonded to the active semiconductor layers. Then a reflective electrode was formed on the support substrate. This remedy proved unsatisfactory, however, as the active semiconductor layers and the transparent support substrate gave rise to electrical resistance at the interface therebetween. This resistance made the forward voltage between the anode and cathode of the light emitting device inconveniently high.
A solution to this weakness of the known remedy is found in Japanese Unexamined Patent Publication No. 2002-217450. This prior patent application teaches the creation of a thin, open-worked layer of gold-germanium-gallium (Au—Ge—Ga) alloy on the underside of the active semiconductor layers. The open-worked Au—Ge—Ga alloy layer, as well as those surface parts of the active semiconductor layers which are left exposed by this open-worked alloy layer, is then covered with a layer of aluminum or like reflective metal. To this reflective metal layer is then bonded a baseplate , or mechanical support, of electrically conductive silicon or like material.
The Au—Ge—Ga alloy layer is known to make favorable ohmic contact with semiconductor substrates of AlGaInP or the like, so that it can reduce the forward voltage between anode and cathode. The efficiency of light emission is also enhanced as the reflective metal layer reflects the light that has been radiated toward the support substrate.
However, this second recited prior art device also proved to have its own weaknesses. One of these weaknesses arose in conjunction with the manufacturing process of the device, which involved several heat treatments. Undesired reactions took place as a result of such heat treatments between the reflective metal layer and Au—Ge—Ga regions and the neighboring parts of the active semiconductor layers. The result was a diminution of reflectivity at their interfaces. High-efficiency light-emitting devices were therefore not obtainable with as high a yield as had been expected.
Japanese Unexamined Patent Publication No. 2003-224297 suggests an interposition of an insulating layer between the reflective metal layer and an ohmic electrode. The insulating layer is partly open to permit electrical connection between the metal layer and the electrode. This construction is objectionable because it leads to a drop in reflectivity as the metal layer and the electrode are easy to make an alloying reaction through the open insulating layer.
A further suggestion for higher efficiency of light emission is made by Japanese Unexamined Patent Publication No. 11-4020, proposing an insertion of a current blocking region of an n-type semiconductor between a transparent electrode and p-type ohmic contact layer. Positioned in register with a bonding pad overlying the transparent electrode, the current blocking region is conductive to higher efficiency as it reduces the amount of current flowing in those parts of the active semiconductor layers which do not contribute to light production. Offsetting this advantage, however, is the difficulty of creating the current blocking region, which requires additional steps for LED production, adding substantively to its manufacturing cost.